Cold start backfires

[MilesnMiles]

Had this problem for a while. If I leave the car a while (week or more) it’s difficult to start. Two pumps on the gas and usually no start and backfire/fart thru the carb. Usually rectified by starting with full throttle. Car always starts then.
Once started the car runs perfectly and will idle from stone cold, run down the road perfectly. It’s just a start issue.

Even when hot I can only start the car if I turn the switch and let the car start, any light throttle can cause that backfire agin.

All parts are new, carb, sparks, ignition switch etc..problem only occurs when starting.

[Dave999]

mmm

initial timing?
wet plugs? due to idle mix or choke
low voltage at the coil when the starter is running

whats ya plug gaps? you only need about 25-30 thou.
try 20 and work upwards all it needs is to start and run cleanly under load in the RPM range you use any wider is just a waste of electricity and a strain on the coil insulation

MSDs and such might work well enough with 35-45 but all you are doing is raising the voltage of the spark at the plug to the detriment of the current that flows, and you need both. power is volts x current x time massive volts and tiny current isn't better (big gap). low volts and massive current = longer spark but less of it exposed to the mix ( small gap) (not great either). Nice mid point in gap size is necessary which varies linked to fuel chosen,mixture, CR at that RPM, air temp/density, the dwell and the kind of coil you run.

if you went from 20 thou direct to 45 thou you'd need to reset the timing as you have introduced a delay in the start of the spark
same as if you change electronic ignition, all of them have an in built delay which you are catering for when you set the timing.
So if you have been doing swapsies you may not have the timing you once had.

its hard to start a car with a cam that has massive overlap
and its hard to start a car with very High CR or using a fuel that just doesn't like to burn when cold
but its also hard to start a car if the coil expects 12 volts and only gets 7 or 8 when the starter motor is running, smaller plug gaps will mask that to a certain extent.

if you run a mini starter and still have the ballast resistor and 8 volts coil. run a wire from the switched pin on the mini starter to the coil end of the ballast.

that will switch battery direct power to the coil when the starter is running and it will switch off and allow ballast resistor power (8-9 Volts) when the starter stops. (this is for a standard mopar ballast resistor coil) makes little odds if you have a 12 volt coil

Dave

[MilesnMiles]

Thanks for your efforts Dave, always appreciate some theory. It certainly feels like the spark is arriving ‘too late’ ( the delay you refer to) as the car fires a bit almost as I turn the switch off.
Il be honest, whilst I’ve fitted new ignition switch/wiring, new coil, new ballast I haven’t actually checked the plugs for a while.
I’ll pull those and gap them down as you say and see how it goes.
I removed the entire choke assembly yesterday, never seemed to work properly from new.

Compression is 9.7.1,cam isn’t huge but there is some overlap 224/230@50. I do have a mini starter.

Cheers,
Miles
Ps the kicker is that once running the car is immediately fine, runs perfect from stone cold; it’s just the starting.

[MilesnMiles]

Time for an update. Dave spotted the cause as usual. I pulled the plugs and all were wet and fuel soaked, surprised it ran at all.
Cleaned and dried all plugs, recapped them as some had big gaps. Advanced initial timing a touch. Pressed throttle twice (no choke as I removed it) and it fired right up and idled. Got it up to temp, reset the four corner idle until it leaned out too much and then back a touch.
Can’t road test as no MOT, but punched the throttle and got the tyres to bark on my drive.
Parked her up. Most satisfying. Let’s hope it does the same next time I go to start it.

[lough3969]

That is satisfying... now get the MOT done...